1. Field of Invention
The present invention relates to a method for improving startability by using electric continuously variable valve timing (CVVT) control. More particularly, the present invention relates to a method that improves startability of vehicles equipped with an electric CVVT device, wherein a cam is located at a position just before a high pressure pump starts to be pressed.
2. Description of Related Art
The optimum opening/closing times of intake/exhaust valves can be generally different according to engine speed, engine load, etc. Therefore, a control technique for appropriate valve timing has been developed, wherein rotations of a camshaft are not determined according to the rotations of a crankshaft but are controlled according to a driving condition of an engine in order to get a predetermined displacement. This is called variable valve timing (VVT).
A continuously variable valve timing (CVVT) is one of variable valve timing methods, wherein valve timing can be controlled by an arbitrary value in predetermined ranges.
FIG. 1 is a cross-sectional view of a general camshaft (100) and a high pressure pump, wherein high pressure is exerted only when a lobe 30 presses a piston 12 of the high pressure pump 10.
Therefore, starting points for pressure increase are different according to positions of the camshaft 100, which leads to a starting delay. In spite of starting at the same time, pressure rising points start at different times because the camshaft 100 meets the lobe 30 of the high pressure pump 10 according to the position of the camshaft 100. As such, a pressure rising time is delayed.
A general CVVT control is fulfilled, as shown in FIG. 2 and FIG. 3, by a CVVT device mounted on the camshaft 100. In case of a dual CVVT, the control is performed by an intake CVVT 140 mounted on a cam 120 at an intake side and an exhaust CVVT 150 mounted on a cam 120 at an exhaust side.
Then, an angle calculation is performed by a rotation speed/load map input into an engine control unit (ECU) 60 when a base profile formed at the cam 120 supplies a phase difference.
If an oil control valve (OCV) 40 is supplied by an electric duty in order to rotate the camshaft 100, the OCV provides an advance chamber 142 or a retard chamber 144 in the CVVT device with high-pressure oil supplied into an oil pump attached to a driving system of an engine, and the camshaft 100 rotates in an advance or retard direction according to the amount of the supplied oil, so the cam profile of the CVVT device moves left or right.
Therefore, the CVVT device rotates much more than a predetermined cam profile in an advance or retard direction (for example, ±45 deg), so a valve overlap is generated.
A conventional mechanical CVVT drives the oil pump for the above operation and raises the hydraulic pressure of an engine and uses the hydraulic pressure, so the CVVT device cannot be moved during a low RPM condition (low hydraulic pressure condition) or starting off.
Also, in a case of a gasoline direct injection (GDI) engine, the GDI engine injects fuel at a higher pressure than in the conventional multi-position injection (MPI) engine and expedites fuel atomization, so an optimum air/fuel ratio can be obtained in spite of relatively little fuel. Further, the fuel is compressed as the drive cam 120 for high pressure pump rotates, the fuel flowed into the high pressure pump is compressed, and necessary rail pressure for injecting is obtained by repeated compression.
However, in a startability aspect, in order to form sufficient pressure at a low temperature, cranking time is long, so a much longer time compared with a conventional engine is needed.
That is, three or four lobes 30 for high pressure pumping are processed at the camshaft 100 according to the layout condition, and engine speed is generally low in a cranking interval and the high pressure pump 10 needs longer time to meet the lobe 30, so longer time is needed for the entire starting process.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.